1. Field of the Invention
The invention relates to overshots employed in the handling and retrieval of "drop" type well survey instruments that are employed in directional drilling.
2. Description of the Prior Art
Directional drilling is currently widely utilized for the efficient exploitation of oil fields, but requires great accuracy in drill orientation as to both azimuth and inclination or drift angle. Thus, a requirement of directional drilling is that the azimuth and inclination angles of the drill be monitored, at least periodically, by a downhole survey instrument. One general type of survey instrument is the "drop" type that is adapted to be dropped down through the drill string to land proximate the bottom, and this may be either a "single shot" or "multiple shot" directional survey instrument depending upon the number of timed photographic shots that are taken of the instrumentation.
The drop type survey instrument is contained in a survey barrel assembly which is made up in a survey instrument string which usually contains non-magnetic spacer bars above and below the survey barrel assembly, and which terminates at its upper end in a spear point that is adapted to be releasably latched or locked to an overshot that is suspended from a wire line or other flexible line in a derrick for lowering of the survey instrument string into the upper part of the drill string in preparation for drop, and for retrieval of the survey instrument string from the drill collars as the string is being pulled from the well after the survey instrument readings have been photographically recorded by the single or multiple shots.
The overshot is required to have a downwardly opening receptacle with spring-loaded locking fingers that will automatically latch or lock over the enlarged tip of the spear point when the latter is engaged in the receptacle. Manual release means must be externally accessible along the sides of the overshot.
Historically, the basic problem in the art has been to achieve a strong, positive radially inwardly directed locking force of the gripping fingers over the spear point, with strong and reliable working parts, but within a very narrow outer diameter limitation for the overshot. The typical survey instrument string has an O.D. of only 13/4 inch for freedom of movement when it is dropped down through the drill string and when it is later retrieved from the drill collars. Correspondingly, it is desirable that the overshot have an O.D. along its length no greater than that of the survey instrument string, i.e., no greater than 13/4 inch.
Not only is the outer diameter of the overshot thus strictly limited, but also the inner diameter of the downwardly opening receptacle part must be as large as possible so that the engaged upper end part of the spear point can have a sufficient diameter to be reliably strong.
The original survey instrument overshot art is the Reed overshot which has gripping arms arranged much like a pair of ice tongs that are biased toward the engaged position by a pair of peripherally exposed leaf springs. The Reed device had a number of serious problems. A basic problem was that it had too large a diameter at the gripping part, so that when it was lowered down into the drill string for retrieval of the survey instrument string it could not get through restricted regions of the drill string. If the gripping part were made diametrically smaller, then the engaged part of the spear point would have had to be so thin that it would be too weak. The exposed springs had a cantilever type action which limited the amount of gripping force that was available. Another, serious problem with the exposed leaf springs was that if the tool got cocked in a narrow region or turn in the drill string, the exposed springs could be accidentally depressed radially inwardly so as to release the gripping part and allow the instrument string to inadvertently drop to the bottom.
One attempt to overcome these problems of the Reed device was the Kuster overshot. This device employed a pair of gripping arms which were loosely floatingly held in a transverse slot and biased downwardly against cam ramps to their gripping positions by means of an external, peripheral coil spring. While the cam ramp gripping action is a good type of action for providing good radial inward gripping force within a relatively narrow radial thickness of the tool, the external coil or helical spring on the Kuster tool caused problems. One such problem was the fact that the external helical spring had to be threaded over a series of external projections on the tool during assembly, which made assembly difficult. A similar difficult unthreading was required for disassembly. In order to permit the external spring to be threaded into its operative position during assembly, the spring had to be a relatively weak one, which limited the amount of gripping force available in the tool. Also, since the two gripping arms independently floated against diametrically opposite sides of the spring, the arms became readily axially displaced relative to each other, so that positive and simultaneous locking and unlocking was not assured with this tool. A still further problem with the Kuster type overshoot was that in addition to the externally exposed helical spring, the tool had series of radial projections or irregularities which could catch on some obstruction.